Useful petroleum products, such as gaseous and liquid fuels, are derived from crude oil by means of one or more petroleum refining techniques. Crude oil is made up of a diverse mixture of hydrocarbons and other compounds which can vary widely in molecular weight and can boil over a rather wide range. Some crude oils contain as much as up to 60 volume percent or more of compounds that boil at temperatures above about 343.degree. C. (650.degree. F.). Such high-boiling components of crude oil are unsuitable as consituents of gasoline or other liquid hydrocarbon fuels. One of the petroleum refining techniques for handling such high molecular weight, high boiling compounds is the fluid catalytic cracking (FCC) process, which cracks or breaks down the high-molecular weight molecules into smaller molecules which boil over a more appropriate boiling range. Today, after many refinements, the FCC process has reached a highly advanced state and many modified forms and variations have been developed. This has culminated in the cracking of a restricted boiling range hydrocarbon feedstock in a riser reaction zone under cracking conditions and at an elevated temperature in contact with a fluidized cracking catalyst that is suspended in the feedstock being cracked, such cracking conditions providing at the riser outlet a temperature in the range of about 510.degree. C. (950.degree. F.) to about 593.degree. C. (1100.degree. F.).
Raw crude oils and certain fractions thereof contain a variety of components which will deposit troublesome deactivating materials on a catalyst, when such components contact the catalyst. This occurs when such oils and/or fractions are catalytically cracked. It is essential that as much as possible of these deactivating materials be removed from the feed prior to its being cracked. However, only a portion of such troublesome deactivating materials can be removed economically from either the crude oil and its fractions or the catalyst, in this case, the fluidized cracking catalyst. Listed among such troublesome deactivating materials are coke precursors, such as asphaltenes, polynuclear aromatics, and the like; heavy metals, such as nickel, vanadium, iron, and copper; lighter metals, such as sodium and potassium; sulfur; and nitrogen. Desalting operations, which are used normally to pretreat crude oil or its fractions that will be used as feed for the FCC process, can remove essentially all of the lighter metals.
On the other hand, coke precursors, such as asphaltenes and polynuclear aromatics, tend to break down and form hydrocarbonaceous deposits or coke on the catalyst during the cracking operation. Such deposits impair further contact of the hydrocarbon feedstock with the active catalytic sites of the catalyst and, hence, reduce conversion. Moreover, any heavy metals in the feed being treated transfer almost quantitatively from the feedstock to the surface of the catalyst, blocking active sites and unfavorably altering the nature of its catalytic effect upon the feedstock. For example, vanadium tends to form fluxes with certain components of commonly used FCC catalysts, lowering the melting point of portions of the catalyst particles. The catalyst is poisoned by accumulations of vanadium and other heavy metals, particularly nickel. Such metals tend in varying degrees to promote hydrogenation, dehydrogenation, and aromatic condensation, resutling in excessive production of carbon and gases with consequent impairment of the yields of liquid fuel components.
In recent years, as the petroleum industry began to suffer from a lack of crude availability as to quantity and quality accompanied by increasing demand for gasolines having increased octane values, the supply situation changed from a surplus of light, sweet crudes to a tighter supply having an increasing amount of heavier crudes containing higher amounts of sulfur and nitrogen. Many of such heavier crudes also contained much higher concentrations of metals and coke precursors or carbon formes, along with increased amounts of asphaltic components.
The need to process heavier and less desirable crudes caused the petroleum industry to search for and provide processing schemes which could utilize such heavier crudes in producing gasoline and other liquid fuel products. The literature has described many of these processign schemes. For example, various proposals in the prior art involve the treating of a heavy oil feed to remove the metals therefrom prior to subjecting the feeds to cracking. Such treating involves hydrotreating, solvent extraction, and complexing with Friedel-Crafts catalysts, but such techniques nullify the refining costs and are criticized as being unjustified economically in the present environment for crude oil availability. Another proposal involves a combination cracking process comprising separate "dirty oil" and "clean oil" processing units. In yet another proposal, residual oil is blended with gas oil and the quantity of residual oil in the mixture is controlled relative to the equilibrium flash vaporization temperature at the bottom of the riser-type cracking reaction zone that is employed in the process. In still another proposal, the feed is subjected to a mild preliminary hydrocracking or hydrotreating operation before it is introduced into the cracking unit.
Although much time, effort, and money have been expended and although some of the above proposals do overcome one or more of the difficulties involved in the conversion of such heavy oil and residual oil fractions, there is a scarcity of techniques for handling such heavy oil fractions, which techniques are both economical and highly practical in terms of technical feasibility. Many hydrocarbon oils, such as crude oils or crude fractions or, in fact, other heavy oil fractions that contain a relatively large amount of nickel and/or other heavy metals exhibit deleterious behavior as described. Such oils not only contain the heavy metals but also relatively large amounts of coke precursors or carbon formers. These heavy oil feedstocks are referred to herein as carbo-metallic oil feeds and represent a particular challenge to the petroleum refiner in his attempt to achieve economic conversion thereof to more useful fuel products comprising gasoline, light cycle oil products and heavy crude oil products. In some cases, various crude oils are relatively free of such high-boiling carbon precursors and coke formers, or heavy metals, or both.
Generally, the troublesome components of crude oil are, for the most part, concentrated in the highest boiling fractions of the crude oil. In view of this, the problems of Conradson carbon coke precursors and the accumulation of heavy metals have been avoided largely by sacrificing some liquid fuel yield that would be potentially available from the highest boiling vacuum bottom portions of crude oils. Since those fractions of crude oil which boil at a temperature within the range of about 343.degree. C. (650.degree. F.) to about 538.degree. C. (1100.degree. F.) is relatively free of heavy metal and Conradson carbon comtamination, such fraction is made a part of the gas oil feedstock to conventional FCC units. Vacuum bottoms material is not included in such feedstock. In general, feedstock comprising atmospheric gas oil and vacuum gas oil (VGO) is generally prepared from crude oil in a two-step technique. The atmospheric gas oil fraction is removed by distillation or the middle distillate boiling below about 316.degree. C. (600.degree. F.) or about 343.degree. C. (650.degree. F.) at atmospheric pressure is distilled from the crude oil. Then vacuum gas oil boiling from about 316.degree. C. (600.degree. F.) up to about 538.degree. C. (1,000.degree. F.) or 552.degree. C. (1,025.degree. F.) end boiling point is separated by vacuum distillation. Gas oil boiling above 316.degree. C. (600.degree. F.) obtained from atmospheric distillation and/or gas oils obtained by vacuum distillation are used as the feedstock for the conventional gas oil FCC processing.
The heavier vacuum resid or vacuum bottoms product obtained from crude oil distillation is normally employed in a number of other ways, for example, such as for the production of asphalt, residual fuel oil, number 6 fuel oil, or marine Bunker C fuel oil. Today it is believed that such vacuum resid represents a great waste of the potential value of this bottom portion of the crude oil, particularly in light of the great effort and expense which the art has been willing to expend in an attempt to provide the relatively similar materials from other sources, such as shale oils and coal.
Generally, the coke-forming tendency of a hydrocarbon oil can be ascertained conveniently by determining the amount of carbon (wt. %) remaining after a sample of that hydrocarbon oil has been pyrolized. Such carbon value is accepted by the industry as a measure of the extent to which a given oil tends to form non-catalytic coke, when such oil is employed as a feedstock in a catalytic cracker. The Conradson Carbon ASTM Method D189 and the Ramsbottom Carbon ASTM Test No. D524-76 are accepted tests for measuring the coke producing tendencies of oils. In the conventional gas oil FCC operation, Conradson carbon values that are generally less than 2 and Ramsbottom carbon values of about 0.1 to 1.0 indicate an acceptable gas oil feed.
As the trend for refining the heavier fractions of crude oils and crude oils transpires, more processes will be developed to treat such higher boiling feedstocks. Among these is the process described in U.S. Ser. No. 413,870 by Miller et al. In such a process, a crude oil is separated into its various fractions by atmospheric distillation and the resid fraction from that distillation step is subsequently distilled in a vacuum distillation tower. The vacuum gas oil produced in such a process is then hydrogenated and can be conveniently combined with a portion of the vacuum resid fraction and optionally with atmospheric gas oil and the resulting mixture employed as a feedstock in a riser-type reaction FCC unit. Moreover, a processing technique is described by Walters et al, in U.S. Ser. No. 617,764 for the combustion removal of high levels of carbonaceous material deposits. Such processes are employed to treat hydrocarbon feedstocks which have Conradson Carbon values in the range of 2 to about 12 and Ramsbottom Carbon values above about 1.0. Such feeds provide a substantially greater potential for coke formation than that which is more usually obtained with feedstocks for gas oil FCC units, which feedstocks have Conradson Carbon values of less than 2.
The more conventional prior art FCC practice involves feedstocks for FCC processing that have heavy metal contents limited to a relatively low value, e.g., about 0.25 ppm Nickel Equivalents (nickel plus vanadium) or less. Those feedstocks which provide the Conradson Carbon values in the range of 2 to 12 in general contain a metal contamination concentration that is in excess of the 0.25 ppm Nickel Equivalents value. Therefore such a feedstock has significantly greater potential for providing a more rapidly accumulating on and poisoning of the catalyst being employed beyond the economic recovery thereof.
In the prior art conventional FCC practice, the metal content of the catalyst is maintained at a level which may, for example, be in the range of about 200 ppm to about 600 ppm Nickel Equivalents. However, the processes employing the heavier feedstocks are concerned with the use of catalysts that have accumulated a rather substantial amount of metals and which, therefore, have a much greater than normal tendency to promote undesired reactions of dehydrogenation, aromatic condensation, gas production, or coke formation. Normally, a metals accumulation of 1,000 ppm to about 3,000 ppm is regarded as quite undesirable in FCC processing.
It is well known that the higher the molecular weight is in a hydrocarbon feedstock, the higher will be the Conradson Carbon value of the feed and, consequently, the higher will be the deposition of hydrocarbonaceous material on the fluidized solid particles. Those feeds comprising components that boil above a temperature of about 552.degree. C. (1,025.degree. F. ) or 566.degree. C. (1,050.degree. F.) and comprising the vacuum resid fraction of a hydrocarbon feedstock will increase the deposition of the carbonaceous material on the fluidized solid particles during the conversion of residual oil. Such deposition of carbonaceous material comprising hydrogen deactivates the catalyst particles. Such carbonaceous material is referred ro simply as coke. In the typical FCC process, the deposited hydrocarbonaceous material is removed from the catalyst by combustion in a separate regeneration vessel provided for that purpose.
It is well known that an increase in the level of coke deposition on fluidized catalyst particles will increase the combustion temperature that is encountered, unless appropriate precautions are taken. Such increase in combustion temperature can be controlled by reducing the catalyst circulation rate, by reducing the concentration of oxygen in the combustion supporting gas, by using higher temperature particles, by providing indirect heat exchange means within the bed of solids in the combustion zone, and combinations thereof. The patents of Medlin, et al., U.S. Pat. No. 2,819,951; McKinney, U.S. Pat. No. 3,990,992; and Vickers, U.S. Pat. No. 4,219,442, disclose fluid catalytic cracking processes using dual combustion zone regenerators with cooling coils in particularly the second regeneration zone. The use of catalyst coolers, which are external to the regeneration or coke combustion zone, is also known in the prior art. For example, please see Harper, U.S. Pat. No. 2,970,117; Owens, U.S. Pat. No. 2,873,175; McKinney, U.S. Pat. No. 2,862,798; Watson, et al., U.S. Pat No. 2,596,748; Jahnig, et al., U.S. Pat. No. 2,515,156; and Berger, U.S. Pat. No. 2,492,948. In addition, Walters, et al., in U.S. Ser. No. 617,764 depart from the prior art by providing catalyst regeneration systems that are considered more suitable for the combustion removal of high levels of carbonaceous material deposits of residual oil conversion from fluid particles and/or catalyst particles in temperature-controlled environments contributed in part by the special design of the regeneration system that is employed in combination with an external catalyst or fluid particulate sorbent cooler. Such cooler design can be used in conjunction with one or more stages of catalyst regeneration. In this cooler design, a plurality of horizontal and vertically-spaced apart gas distributor means into one or more chambers in the regeneration system provides for passing fluidizing gas into the chambers in order to contact said fluidizing gas with solid particles flowing downwardly through said chamber. However, certain disadvantages are associated with this plurality of horizontal and vertically-spaced apart gas distributor means, which comprises a plurality of horizontal and vertically-spaced apart conduit means. The high temperature in the cooler vessel, as high as 760.degree. C. (1,400.degree. F.), and the downward pressure of the fluidizable particulate solids passing downwardly around the horizontal conduit means results in deformation of the conduit means. Deformation of the conduit means leads to impingement of the fluidizing gas emanating from the apertures of the conduit means upon adjacent cooler tubes, causing localized erosion of the cooler tubes.
The present invention is directed to an improved apparatus for cooling fluidized solid particles. The improvement comprises the use of vertical aeration lances, conduits, or tubes. Such vertical aeration lances, conduits, or tubes aviod, or at least minimize, high temperature deformation of such lances, conduits, or tubes and, hence, prevent the resulting localized erosion of cooler tubes.